The present invention relates to a system for controlling the amount of fuel vapor to be purged into a carburetor from a charcoal canister of a fuel-vapor recovery system and also for controlling the air-fuel ratio of an air-fuel mixture to be supplied by the carburetor for internal combustion engines on automobiles.
There is known a fuel-vapor recovery system associated with an automotive engine for temporarily storing fuel vapor from a fuel tank and a carburetor float bowl into a charcoal canister when the engine is shut off and for purging the stored fuel vapor into the carburetor when the engine is in operation. The fuel vapor stored in the charcoal canister is purged therefrom under vacuum developed in the intake manifold when the engine is started. The fuel-vapor recovery system thus serves to prevent fuel vapor from being lost from the fuel tank at an atmospheric temperature while the automobile is being parked and also from the carburetor float bowl at an elevated engine temperature when the engine is stopped. Normally the amount of fuel vapor produced at an atmospheric or elevated temperature is relatively small, and therefore full fuel-vapor flow from the canister does not substantially affect engine operation even if it is not controlled according to engine operating conditions.
However, one recent fuel-vapor recovery system also includes an on-board charcoal canister of a large fuel-vapor storage capacity added for storing a much greater amount of fuel vapor developed in the fuel tank when fuel is supplied under high pressure to the fuel tank from a fuel supply nozzle at a gasoline station. The amount of fuel vapor stored in, and hence purged from, the on-board charcoal canister is quite large, and, unless properly controlled, would upset the air-fuel ratio of the air-fuel mixture supplied to the engine, thereby adversely affecting the emission reduction capability of the engine and the exhaust gas discharged from the engine. Where the engine is equipped with an air-fuel ratio control system for optimizing the air-fuel ratio of the air-fuel mixture based on a detected density of oxygen contained in the exhaust gas, an air-fuel ratio disturbed by the purged fuel vapor flow would be apt to prevent the air-fuel ratio control system from properly controlling the air-fuel ratio.
To minimize the adverse effects on the engine while allowing a sufficient fuel-vapor flow from the on-board canister to the carburetor, it is necessary, in combination with an air-fuel ratio control system, to control the purged amount of fuel vapor from the on-board canister in proportion to the amount of air drawn by the engine. If the fuel vapor were not controlled in proportion to the amount of drawn air, then the control of the air-fuel ratio by the air-fuel ratio control system would not be performed correctly. Furthermore, if the purged fuel vapor were to be varied in quick response to the start of an air-fuel ratio control mode or changes in the amount of air drawn by the engine, then the actual air-fuel ratio would deviate widely from an air-fuel ratio setpoint due to a response delay caused by the closed feedback loop of the air-fuel ratio control system, and hence it would take a long period of time before the actual air-fuel ratio would settle into the air-fuel ratio setpoint.
As disclosed in Japanese Laid-Open Publication No. 57(1983)-20529, the air-fuel ratio control system typically includes an O.sub.2 sensor for detecting a density of oxygen in the exhaust gas discharged from the engine and an electronic control unit (ECU) responsive to a signal from the O.sub.2 sensor for operating an actuator to control the density of an air-fuel mixture in the carburetor so that the air-fuel ratio of the air-fuel mixture will be equalized to a stoichiometric air-fuel ratio. Specifically, the actuator controls the air-fuel mixture density by varying the cross-sectional area of a passage for introducing air therethrough into in the carburetor or of a passage for supplying fuel therethrough into the carburetor. The information derived from the output signal generated by the O.sub.2 sensor is indicative of only whether the air-fuel mixture is richer or leaner than the stoichiometric air-fuel ratio, but not of how wide the actual air-fuel ratio deviates from the stoichiometric air-fuel ratio. Therefore, the ECU operates to vary the air-fuel ratio to a predicted extent in the PI control mode in the direction that is determined by the binary output signal from the O.sub.2 sensor. In addition, various engine operating conditions are detected by an engine condition detector for enabling the ECU to control the air-fuel ratio at a level optimum for the engine operation.
The time required for the O.sub.2 sensor to respond to the oxygen density, the time required for the ECU to effect given calculations, and the time required for the actuator to operate, result in the response delay, as described above, of the closed feedback loop of the air-fuel ratio control system.